Radio-frequency (RF) amplifier circuits employ impedance matching networks (also known as impedance transformers) in order to optimize the transfer of power between an RF source, RF amplifier transistors and a load. In the case of very wide bandwidths, these impedance transformers are generally produced with the aid of transmission lines that often take the form of interconnected coaxial cables.
For example, wideband RF amplifiers use, notably in the case of high powers, transistors connected in push-pull, each having a signal input and a balanced power RF output. Their RF inputs and outputs have impedances much lower than that of the usual 50Ω transmission lines. The use of impedance transformers at the input and output of the amplifier transistor therefore proves necessary to obtain an optimum transfer of power.
FIG. 1 is a diagram of one embodiment of a typical push-pull RF amplifier stage using such transformers.
The amplifier stage of FIG. 1a includes two amplifier transistors A and B connected in push-pull, an input transformer Te and an output transformer Ts with symmetrical inputs and outputs to adapt the input and output impedances, respectively, of the amplifier stage, by means of an input balun Be and an output balun Bs, to the low input and output impedances of the transistors A and B. The input balun Be and the output balun Bs provide respective connections between the unbalanced input E and output S of the amplifier and the balanced accesses of the transformers.
A generator with an impedance of 50Ω applies an RF signal to be amplified to the unbalanced input E of the input balun Be forming the input of the amplifier. The unbalanced output S of the output balun Bs forming the output of the amplifier stage is applied to a 50Ω load.
Note that the term “balun” is a contraction of the words BALanced and UNbalanced.
FIG. 2a is a diagram of one example of a prior art coaxial line impedance transformer.
The transformer in FIG. 2a effects impedance transformation from a high-impedance access Eh to a low-impedance access Eb, and in this example the impedance of the access Eh is 50Ω and the impedance of the access Eb is 12.5Ω. The transformer in FIG. 2a includes two coaxial lines L1, L2 with characteristic impedance Zc=25Ω each including an internal conductor Ci and an external conductor Ce surrounding the internal conductor.
The two lines L1, L2 are connected in series on the high-impedance access Eh side and in parallel on the low-impedance access Eb side. To this end, on the high-impedance access side of the transformer, the external conductors Ce of the two lines L1, L2 are connected together and possibly to a reference potential, for example ground M. On the low-impedance access side of the transformer, the internal conductor Ci of one of the lines is connected to the external conductor Ce of the other line and vice versa. RF signal input and output are effected in balanced mode via the two internal conductors Ci of the coaxial lines.
The impedance transformation ratio remains fixed, theoretically equal to 4 in the case of the transformer in FIG. 2a. 
The use of ferrite blocks around the coaxial lines (not shown in the figure) enables the bandwidth of the transformer to be widened at the low-frequency end.
FIG. 2b is a simplified layout diagram of an RF amplifier stage.
The amplifier stage includes, on a printed circuit 10, an integrated circuit 20 with two transistors to be connected in push-pull, an input transformer T1 and an output transformer T2 as in the FIG. 2a diagram.
The input transformer T1 includes two lines Le1 and Le2 connected in series on its high-impedance access Eh side and in parallel on its low-impedance access Eb side, as shown in FIG. 2b. The internal conductors Ci of the two lines connect the inputs e1, e2 of the two transistors in the integrated circuit 20 via a matching unit 24.
The output transformer T2, constructed like the input transformer T1, includes two coaxial lines Ls1 and Ls2 and is connected by its low-impedance access Eb to the outputs s1, s2 of the transistors in the integrated circuit 20, its high-impedance access Eh being intended to be connected to a load that is not shown in the figure.
The lines Le1, Le2, Ls1, Ls2 of the transformers T1, T2 are coiled here to reduce their overall size within the amplifier.
The FIG. 2b type embodiment using coiled coaxial line transformers is still of the hand-wired variety, which impact on the production cost and the overall size (above all in length) of the amplifier stage.
To limit the overall size of RF impedance transformers, some prior art embodiments use printed circuits to replace the coaxial lines. There are very many such embodiments and some are commercially available, but for modest bandwidths and above all modest powers.
Only two particular examples will be cited, on the basis of which the advantages of the proposed invention will be described.
FIGS. 3a and 3b are views in cross section and in elevation of a prior art embodiment of an impedance transformer described by Georg Boeck in 0-7803-9342-2/05/$20.00 © 2005 IEEE.
The impedance transformer in FIG. 3a is produced on a multilayer printed circuit 30 with four metalized layers integrating rectangular microstrip type lines.
FIG. 3a is a view in cross section of the printed circuit with four layers in an area including lines with an impedance ZL of 25Ω and lines with an impedance ZL of 50Ω.
These rectangular coaxial lines may have impedances ZL of 25Ω or 50Ω depending on the chosen disposition, thus enabling integration into a 50Ω circuit of the transformer that uses 25Ω lines.
FIG. 3b is a plan view of the impedance transformer from FIG. 3a. The microstrip lines are interleaved in a spiral in order to reduce their overall size, which necessitates numerous crossings of lines compromising performance and power rating. Vias 32 interconnect the metallizations of the various layers of the printed circuit.
The embodiment of FIGS. 3a and 3b is suitably only for uses with a very low signal level because of the spiral topology used, compromising performance, notably in terms of insertion losses.
FIG. 4a is a perspective view of another prior art embodiment of an impedance transformer. FIG. 4b is a view in cross section of the transformer from FIG. 4a. 
In the FIG. 4a embodiment the problem of many crossing lines of the embodiment of FIGS. 3a and 3b does not arise because of an embodiment topology based on a single pair of tracks forming lines bent to a U-shape. To this end, the transformer in FIGS. 4a and 4b includes a double-sided substrate 40 having metallization on both faces forming microstrip type lines L1, L2 interconnected by conductive transitions between faces.
The FIG. 4a embodiment includes:                a conductor (or metallization) 308 on one face 44 of the substrate 40 and another conductor 316 facing the first on the other face 46 of the substrate 40 to form the first U-shaped microstrip line L1,        a second microstrip line L2 including one conductor (or metallization) 304 on the face 44 of the substrate and another conductor 316 facing the first on the other face of the substrate 40 to form the second U-shaped microstrip line L2 symmetrical to the first L1.        
At one end 50 of the substrate, the free ends of the conductors 308, 304 on the same face 44 of the substrate 40 of the two lines L1, L2 form serial input ports 5, 3 (high-impedance accesses), and the ends of the conductors 316, 312 on the other face 46 of the substrate 40 are connected together to form a port 4 or common point.
At the other end of the substrate 52 opposite the first end 50, the end of the conductor 304 of the line L2 on the face 44 of the substrate 40 and the end of the conductor 316 of the line L1 on the other face 46 of the substrate 40 are connected together to an output port 2 and the end of the conductor 312 of the line L2 on the other face 46 of the substrate and the end of the conductor 308 of the line L1 on the face 44 of the substrate are connected together to a port 1, the ports 1 and 2 forming the parallel low-impedance access of the transformer in FIG. 3a. 
This other embodiment of FIGS. 4a and 4b, although it enables a good power rating, has the drawback of being bulky and furthermore the impedance transformation ratio remains fixed (theoretically equal to 4).